Livestock confinement facilities generate large amounts of animal waste that can create serious environmental and human health concerns. For example, animal waste constituents such as organic matter, nitrogen, phosphorus, pathogens and metals can degrade water quality, air quality, and adversely impact human health. Organic matter, for example, contains a high amount of biodegradable organics and when discharged to surface waters will compete for, and deplete the limited amount of dissolved oxygen available, causing fish kills and other undesirable impacts. Similarly nutrient loading from nitrogen and phosphorus can lead to eutrophication of surface waters.
The annual accumulation of organic waste in the world is immense. There are approximately 450,000 Animal Feeding Operations (“AFOs”) in the United States. Common types of AFOs include dairies, cattle feedlots, and poultry farms. A single dairy cow produces approximately 120 pounds of wet manure per day. The waste produced per day by one dairy cow is equal to that of 20-40 people. If properly stored and used, manure from animal feeding operations can be a valuable resource.
Anaerobic digester technology is a manure management technology capable of alleviating environmental concerns through waste stabilization, odor reduction, pathogen control and greenhouse gas entrapment and mitigation, while producing a renewable source of heat and power (US-EPA, 2005). Adoption of anaerobic digesters on US dairies is growing but still slow with numbers insufficient to meet the agreement between the US and its dairy industry to reduce climate impacts from dairies by 25% by 2020 (USDA, 2010).
Anaerobic digesters utilizes microorganisms to breakdown the organic carbon in manure in the absence of oxygen producing a biogas mainly composed of methane (CH4), and contaminants including carbon dioxide (CO2) and hydrogen sulfide (H2S). Biogas derived from the anaerobic digestion (AD) of dairy manure consists of methane (CH4) (55-65%) and contaminants including carbon dioxide (CO2) (30-45%), low concentrations of hydrogen sulfide (H2S) (300-4,500 ppm) (Liebrand and Ling 2009).
These contaminants, without pretreatment, limit the use of biogas to combined heat and power (CHP) on-site, whereas many economic assessments have suggested that a higher value use is possible by purifying CH4 to meet natural gas regulations or purifying and compressing it for use as a vehicle fuel.
H2S, which is produced by the breakdown of proteins and other sulfur containing compounds during hydrolysis, is detrimental to an internal combustion engine as well as to the environment and human health. Even at low concentrations, H2S has an unpleasant odor and can be life threatening (Speece 1996). Consequently, the recommended industrial exposure limits are from 8 to 10 ppm for 8 hours a day per week (Horikawa, Rossi et al. 2004). Furthermore, this contaminant is highly non-desirable in energy-recovery processes because it converts to unhealthy and environmentally hazardous sulfur dioxide (SO2) and sulfuric acid (H2SO4) (Abatzoglou and Boivin 2009). H2SO4 is neutralized by the alkalinity in the engine oil and requires frequent oil replacement or running the engines rich in fuel to decrease the contact time between the raw biogas and the engine to limit corrosion from the H2SO4 (Fulton 1991). H2S must be reduced to a level less than 4 ppm before direct injection into pipelines (Wise 1981) and completely removed from biogas if it is going to be compressed and used as a vehicle fuel.
CO2 is not detrimental to equipment or human health like other impurities, but because of its comparatively high presence in biogas, it does decrease the energy potential of biogas, due to its inert nature. Due to the presence of CO2 and the lower CH4 content in biogas, the energy potential of biogas is around 612 BTU/scf, whereas natural gas has an energy potential of 1031 BTU/scf (Bothi 2007).
Therefore, methods and apparatuses that can remove H2S and CO2 from biogas are needed and would have a tremendous impact in the industry.